This invention relates in general to an image compression technique and more particularily to one which operates on a binary image of a fingerprint to reduce the size of the image and more particularily to reduce the number of picture elements (pixels) in the image.
For many fingerprint analysis purposes, it is desirable to generate an image which has relatively fine detail. In particular, in certain circumstances it is desired to generate an image from an optical and electronic scan of a finger or fingerprint that is 1,024 by 1,024 pixels. However in downstream processing, the capacity required to handle an image having over one million pixels is prohibitive in terms of cost and also tends to limit the speed of processing to an unacceptably slow level in an access identification system.
Accordingly, it is a major purpose of this invention to provide a technique for reducing the number of pixels involved in providing such an image while enhancing image quality.
When a live finger is scanned optically to provide a finger image or fingerprint image, a relatively high resolution is required to make sure that one picks up the valleys. In particular, it has been found necessary to use a scan which provides an initial image that has 7.5 mils (0.0075 inches) resolution per pixel. In order to obtain this kind of resolution, a picture image of approximately 1,024 by 1,024 pixels has been found useful. The image provided from the optical scan tends to be primarily ridge like in nature. But the relatively small valleys do adequately show. A technique is then required to filter this image so as to provide a resulting image that has approximately a 50:50 ratio ratio between ridge and valley zones. This filtering can be achieved because of the fact that the output from the array of optically responsive diodes that are used in the optical scan have a gray scale value which provides a basis for making a distinction between the high points on the ridges and the low points in the ridges.
Although a picture having approximately one million pixels is required to obtain appropriate image resolution and although a filtering technique has to be employed to convert the optically produced image into an image having a usable approximately 50:50 ratio of ridge zone area to valley zone area, it is desirable from the point of view further downstream processing to provide an image having substantially fewer pixels. The advantages of downstream processing of an image having substantially fewer pixels include the requirement for substantially less storage and more importantly the fact that the processing in verification will be substantially faster. By reducing the number of pixels in the image to one-sixteenth of those in the original input image, storage requirements are cut by a facter of sixteen and processing time is cut by a factor of sixteen.
Accordingly, a major purpose of this invention is to provide an enhanced image with a substantially reduced number of pixels.
However, it is important that the image provided have substantially as much relevant information as does the input image. Accordingly, it is a further purpose of this invention to provide a technique for compressing the size of the image without losing substantial or significant information.
It turns out that by an appropriate compression technique, a certain amount of filtering can occur which will improve the value of the image. Accordingly, a further object to this invention is to provide an image compression technique which creates an enchanced image having sufficient filtering so that downstream processing including such steps as minutia extraction, are improved. It has been found that one result of the application of this invention is that the minutia extraction technique becomes a great deal more stable when applied to the image that is compressed by the technique of this invention than when it is applied to a non-compressed image.